This invention relates to fluids that exhibit substantial increases in flow resistance when exposed to magnetic fields.
Fluid compositions that undergo a change in apparent viscosity in the presence of a magnetic field are commonly referred to as Bingham magnetic fluids or magnetorheological fluids. Magnetorheological fluids typically include magnetic-responsive particles dispersed or suspended in a carrier fluid. In the presence of a magnetic field, the magnetic-responsive particles become polarized and are thereby organized into chains of particles or particle fibrils within the carrier fluid. The chains of particles act to increase the apparent viscosity or flow resistance of the overall materials resulting in the development of a solid mass having a yield stress that must be exceeded to induce onset of flow of the magnetorheological fluid. The force required to exceed the yield stress is referred to as the "yield strength". In the absence of a magnetic field, the particles return to an unorganized or free state and the apparent viscosity or flow resistance of the overall materials is correspondingly reduced. Such absence of a magnetic field is referred to herein as the "off-state".
Magnetorheological fluids are useful in devices or systems for controlling vibration and/or noise. For example, magnetorheological fluids are useful in providing controllable forces acting upon a piston in linear devices such as dampers, mounts and similar devices. Magnetorheological fluids are also useful for providing controllable torque acting upon a rotor in rotary devices. Possible linear or rotary devices could be clutches, brakes, valves, dampers, mounts and similar devices. In these applications magnetorheological fluid can be subjected to shear forces as high as 70 kPa, often significantly higher, and shear rates in the order of 20,000 to 50,000 sec.sup.-1 causing extreme wear on the magnetic-responsive particles. As a result, the magnetorheological fluid thickens substantially over time leading to increasing off-state viscosity. The increasing off-state viscosity leads to an increase in off-state force experienced by the piston or rotor. This increase in off-state force hampers the freedom of movement of the piston or rotor at off-state conditions. In addition, it is desirable to maximize the ratio of on-state force to off-state force in order to maximize the controllability offered by the device. Since the on-state force is dependent upon the magnitude of the applied magnetic field, the on-state force should remain constant at any given applied magnetic field. If the off-state force increases over time because the off-state viscosity is increasing but the on-state force remains constant, the on-state/off-state ratio will decrease. This on-state/off-state ratio decrease results in undesirable minimization of the controllability offered by the device. A more durable magnetorheological fluid that does not thicken over an extended period of time, preferably over the life of the device that includes the fluid, would be very useful.
Magnetorheological fluids are described, for example, in U.S. Pat. Nos. 5,382,373, 5,578,238, 5,599,474 and 5,645,752. These patents mention that phosphate esters, in general, can be used as surfactants in magnetorheological fluids. U.S. Pat. No. 5,645,752 describes a magnetorheological fluid example formulation that includes a polyoxyalkylated alkylaryl phosphate ester.
U.S. Pat. No. 5,271,858 relates to an electrorheological fluid that includes esters and amides of an acid of phosphorus. U.S. Pat. No. 2,751,352 mentions that a magnetic fluid could include antioxidants or antiwear agents such as organic phosphorus compounds with dilorol phosphate, dilauryl phosphite, tributyl phosphate and tricresyl phosphate being listed. U.S. Pat. No. 5,147,573 relates to a magnetic colloid or ferrofluid that includes a surfactant having the general structure R"--R'--R-- YH. Phosphate and thiol are mentioned as possible groups for YH and a secondary amine is mentioned as a possibility for R'.